Hitherto, there has been used an electrochemical reference electrode of a type whose electrode potential does not substantially change in a system in which the ion concentration in the fluid to be experimented upon changes, in particular, the hydrogen ion concentration, and the measuring temperature change. A known reference electrode of this type is exemplified by a saturated calomel electrode, a silver/silver chloride electrode and a hydrogen electrode. Furthermore, in recent years, so-called a hybrid type electrode has been studied in which an organic film made of, for example, polystyrene and polyparaxylylene is formed on a gate insulator film of an MOS (Metal Oxide Semiconductor) type field effect transistor (to be called "an MOSFET" hereinafter).
However, the saturated calomel electrode or the like of the above-described conventional reference electrodes arises a problem in that the size thereof cannot be easily reduced due to the existence of liquid junctions between the reference electrolyte solution and its container.
On the other hand, the MOSFET, in which the gate insulator film thereof is coated with an organic film made of, for example, polystyrene, can be easily miniaturized since it is a solid state device and manufactured by utilizing semiconductor manufacturing technology. However, the MOSFET raises a problem in that its equilibrium potential involves an excessively large drift and raises another problem in that its potential can be easily affected by the ion concentration such as the hydrogen ion.
The silver/silver chloride electrode having an internal fluid chamber in which KCl saturated solution is enclosed usually comprises an electrode in the form of a glass chamber the diameter of which is 2 to 3 cm exhibiting an excellent communication with the external solution. In this case, it is necessary that the internal K.sup.+ and Cl.sup.- ions and the external Cl.sup.- ions can easily communicate with one another. Therefore, a problem arises in that the potential can be changed due to the affection of the contamination of internal fluid and the affection of the external Cl.sup.- ions.
In order to overcome the above-described problems, there have been the following desires:
(1) The flowing out of the internal AgCl and KCl must be prevented as much as possible.
(2) An in-phase structure of Ag/AgCl must be constituted.
(3) The reference potential can be displayed without being affected by the Cl.sup.- ions regardless of the fluid to be experimented (Cl.sup.- included).
An object of the present invention is to provide a stable microminiaturized solid (hybrid) reference electrode which is substantially free from outflow of halide.
Another object of the present invention is to provide a reference electrode which can be easily miniaturized, is not affected by ion concentration, provides a stable potential, has excellent stability and high durability.
In order to overcome the above-described problems, the reference electrode according to the present invention is characterized by comprising: a conductive substrate; and insulating and/or hydrophobic material layer formed by neutron or atomic beam sputtering method and covering the surface of the conductive substrate, the insulating and/or hydrophobic material layer containing silver halide dispersed therein.
The reference electrode according to the present invention is characterized by comprising: a conductive substrate; and a laminate film obtained by laminating alternately a thin layer of silver halide and a thin layer of an insulating and/or hydrophobic material, the laminate layer covering the surface of the conductive substrate.
The reference electrode according to the present invention is characterized by comprising: a field effect transistor; and the insulating and/or hydrophobic material layer containing silver halide dispersed therein or the laminate film, formed on the surface of a gate insulator layer of the field effect transistor, or the insulating and/or hydrophobic material layer containing silver halide dispersed therein or the laminate film, formed on the surface of the gate insulator layer of the field effect transistor via a silver thin layer.
The conductive substrate is a substrate formed by silver or a substrate obtained by forming a silver layer on an insulating body. It is preferable that the silver halide be any of silver chloride, silver bromide, and silver iodide. The silver halide may contain an oxidizing agent selected from potassium ferricyanide, potassium chromate, potassium perchlorate or the like or other halide salt mixed thereto. It may be only a mixture of the oxidizing agent selected from potassium ferricyanide, potassium chromate, potassium perchlorate or the like and the halide salt. It is preferable that the insulating and/or hydrophobic material be any of fluorine organic compound (a fluororesin), polytetrafluoroethylene, aromatic polyparaxylylene and polystyrene.
It is preferable that a hydrophilic layer or an ion permeable layer be formed on the surface of the insulating and/or hydrophobic material layer or the laminate film for the purpose of improving antithrombus characteristics, separating impurity ions or the like and transmitting ions, in particular, transmitting chloride ions. It is preferable that the above-described layer comprises a perfluoro-ion exchange layer such as polyvinyl chloride-polyethylene glycol copolymer, styrene-hydroxyethylmethaacrylate (St-HEMA) copolymer, styrene-hydroxyethylmethaacrylate block polymer, polyurethane, polyvinyl alcohol, polyhydroxyethylmethaacrylate, polyacrylamidegel, and polyperfluorosulfonic acid.
According to the present invention, a miniaturized stable solid (hybrid type) reference electrode which is almost free from outflow of silver halide can be provided.
Specifically,
(1) Since it is covered with an insulating and/or hydrophobic material layer and the neutral atomic beam method is employed, silver halide cannot easily flow out by virtue of the structure of the material designed in the atomic or molecular state.
(2) Since the electrode according to this mode is, in a state of silver halide and Ag (evaporated), applied to the surface of the ITO glass, Ag.sup.+ ions can be always supplied from the substrate. Furthermore, the silver halide is in the form of a land surrounded by tetrafluoroethylene substrate. Therefore, the silver halide/Ag electrode of in-phase structure can be constituted.
(3) The AgCl portion can be easily applied to a miniaturized sensing portion since the above-described method (the neutral or atomic beam sputtering method) is employed. As a result, it can be used as a reference electrode for an ISFET (Ion Selective Field Effect Transistor), and can also be widely used as a hybrid reference electrode for other miniaturized sensors.
When the reference electrode having the above-described laminate film is dipped into water solution, water molecules penetrate a thin layer of the insulating and/or hydrophobic material of the laminate film. Therefore, halide ions of constant concentration generate in the silver halide layer, causing the silver/silver halide in the first layer to generate a certain potential. That is, each of the silver halide layers serves as the reference electrolyte solution and the reference liquid chamber of the conventional reference electrode. Therefore, the above-described reference electrode is dipped into water solution together with the ion electrode so as to use it as an electrode for generating reference potential at the time of measuring ion concentration or the other operations so that the ion concentration is obtained by measuring the difference in the potential between the reference electrode and the ion electrode.
According to the reference electrode according to the present invention, the structure is arranged in such a manner that the laminate film obtained by alternately layering the thin layer of silver halide and a thin layer of an insulating and/or hydrophobic material is applied to the surface of the conductive substrate so that a solid-type reference electrode is formed. Therefore, it can be easily miniaturized, is not affected by pH, chloride ion concentration or the like, provides a constant potential, and has satisfactory stability and high durability.
Furthermore, since the structure is arranged in such a manner that a laminate film obtained by alternately laminating different thin layers is used for the coating, the combination of materials can be easily performed. In addition, since the atomic beam sputtering apparatus can be used for forming the laminate film, the layering can be conducted at room temperature. Furthermore, high dimensional accuracy can be realize at the time of forming a pattern. Therefore, a mass production can be enabled when it is included in a semiconductor manufacturing process. In addition, since the electrode can be operated even if the thickness of the laminate film is less than 1000.ANG., it can be used as a reference electrode to be used integrally with a sensor for use in a miniaturized range of .mu.m or less.
The above-described effects can be further effectively improved by arranging the structure in such a manner that the conductive substrate is formed by silver, the substrate arranged such that silver layer is formed on the surface of the insulating material, the silver halide is selected from any of silver chloride, silver bromide, and silver iodide and the insulating and/or hydrophobic material is selected from polytetrafluoroethylene, aromatic polyparaxylyene and polystyrene.
Other and further objects, features and advantages of the invention will be appear more fully from the following description.